The present invention relates to the mixing of fluid dispersions and more specifically to apparatus and methods for mixing paint disposed in a container.
As is well known, solids in fluid dispersions, such as paint, tend to settle in a downward direction through the force of gravity. Fluid dispersions disposed in containers for commercial sale are typically mixed in the containers before they are used by the purchasers. Many fluid dispersions can be facilely mixed in a container by manually shaking the container. Other fluid dispersions, however, such as paint, are more difficult to manually mix in a container and, thus, are often mixed in the container using a machine that shakes, rotates, vibrates or otherwise moves the container.
A variety of different types of mixing machines are known for mixing fluid dispersions disposed in containers. One type of known mixing machine is disclosed in U.S. Pat. No. 4,134,689 to Ahrenskou-Sorensen, which is hereby incorporated by reference. This type of mixing machine comprises a table having a support surface upon which a container may be placed. A clamping member is movably mounted above the table for clamping the container to the table. A first electric motor is operable to vertically move the clamping member toward and away from the table. The amount of pressure applied by the clamping member to the container is monitored by measuring the current drawn by the first electric motor. When the current drawn by the first electric motor exceeds a certain upper level corresponding to a maximum amount of force, electric power to the first electric motor is either cut off or limited to the upper current level. In this manner, the force applied to the container does not exceed the maximum amount.
When the container is firmly clamped to the table, a second electric motor is operable to vibrate the table and the clamping member to mix the fluid dispersion in the container.
The foregoing type of mixing machine can hold different sized containers, such as a 5 gallon paint container or a 1 gallon paint container. In addition, the mixing machine can hold a plurality of 1 gallon containers, such as a case of 1 gallon containers. Regardless of the type of container or number of containers placed on the table, the clamping member applies the same maximum amount of force to the container(s). Typically, the maximum amount of force is based on the amount of force required to securely hold a 5 gallon container of paint during shaking, which is usually around 900 ft-lbs or more of force. Unfortunately, this amount of force can crush a smaller container that is not as structually strong as a 5 gallon container. For example, the mixing machine will often crush a single 1 gallon container of conventional steel construction if the container has been dented or otherwise damaged. Moreover, the mixing machine would crush a container having an unconventional plastic construction, such as is disclosed in U.S. patent application Publication No. US2001/0025865A1 to Bravo et al., which is hereby incorporated by reference.
U.S. Pat. No. 5,268,620 to Hellenberg discloses an improved mixing machine that addresses the foregoing deficiency of the mixing machine of U.S. Pat. No. 4,134,689. This improved mixing machine has an opto-coupler or shaft encoder for determining the absolute vertical position of the clamping member. The opto-coupler or shaft encoder is connected to a sophisticated control system, which utilizes the position of the clamping member to change the maximum amount of force to be applied by the clamping member. The control system stores a first maximum force, such as for a 5 gallon container and a second maximum force, such as for a 1 gallon container, which is less than the first maximum force. Initially, the control system will limit the force applied by the clamping member to the first maximum amount of force. If, however, the clamping member travels below the height of a standard 5 gallon container without experiencing resistance, the control system determines that a 1 gallon container or other type of small container is present in the mixing machine. In response, the control system will limit the force applied by the clamping member to the second maximum amount of force.
Although the mixing machine of U.S. Pat. No. 5,268,620 solves the force problem for a conventional 1 gallon steel container, it does not solve the force problem for a container having an unconventional plastic construction, such as is disclosed in U.S. patent application Publication No. US2001/0025865A1 to Bravo et al.
Accordingly, there is a need in the art for an improved apparatus and method for mixing fluid dispersions disposed in containers having different sizes and constructions. The present invention is directed to such an apparatus and method.